Studies are continuing on the process of recognition of foreigh antigen by human T cells, particularly with respect to the involvement of HLA gene products. Studies in this project this year include analysis of T cell responses to SB alloantigens and to virus-infected cells. The principal effector mechanism under investigation is cell-mediated cytotoxicity, but proliferation is also used as a measurement of T cell response. Progress has been made in dissecting some of the functional interactions between SB antigen-specific T cells and antigen bearing cells using monoclonal antibodies specific for T cell differentiation antigens. These studies suggest a functional role of the OKT4 molecules in T cell recognition of HLA class II molecules. However clonal analysis of such responses indicates great clonal heterogeneity, the basis for which is under investigation. Studies of T cells responses to measles-virus infected cells demonstrate that despite low frequency of donors whose cells amount strong cytotoxic responses, the responses of those donors are functionally similar to the responses to influenza virus which we have characterized previously. Analysis of influenza-specific responses of donors with the immunodeficiency Ataxia Telangietasia demonstrate severe defects in specific proliferative and cytotoxic responses.